Method of drying inorganic salts and sludges



March 17, 1936.

D J. VAN MARLE METHOD OF DRYING INORGANIC SALTS AND SLUDGES Fiied July10, 1935 p' 77(INV 1 08M I g ENTOR Y n 91.4 620014 A TTORNEY? Patented.17, 1936 v UNITED; STATES RATE I ME'rnon or msfizz zge e san'rs NTorrica Dirk .Lfven Marieiiunaio, N. Ysassignorto p Bufialo Foundry &Machine 00., Buffalo, N. Y., a'corporationof New York Application my 10,1933'. serial No. 519.021

1 Claim. (01. 159-7) This invention relates to a process for dryinginorganic salts, such as sodium sulfate, sodium 'sulfite, ammoniumphosphate,'sodium phos hate,

sodium carbonate, sodium chloride, pot ium salts, calcium chloride,magnesium chloride and sludges and more particularly to a continuousdrying process in which the inorganic salts to I. where ascraper ordoctor blade removes the mabe dried are fed in the form of a solution orsludge to a heated revolving drum from which the dried product isscrapedby a knife.

.V5fl0l18m8t81'i8lS'a1e now being dried, flaked ,or cooled on bronze,steel or cast iron drums.

The drying drums are usually heated by steam and the cooling drumscooled by water; brine or direct expansion. The materials in a liquid orsemi-liquid state are applied to the rotating drum or drums and carriedaround to a fixed point terlal from the drums, and deposits it in asuitable conveyor. v I p The drum dryers as heretofore constructed arenot entirely satisfactory and in fact they are unsuitable for dryingmany inorganic salts and sludges. However, the short period of dryingand the continuous feature of this type of dryer make it very desirableto use for drying such inorganic salts. 7

The dimculty encountered with steel and cast. iron drums is the properremoval of the dried or cooled substance by the knives ordoctor bladesfrom the drum surfaces. 'Itappears that cast iron and steel have'a greatafilnity for and retentive power of inorganic saltsthat are being driedon such surfaces, 'theinorganic productsadhering so tenaciously that itis impossible to completely remove the dried product by the use of am p-"of knife known at present. The dried product on the drum or drumsistherefore only partly removed by the knives, part of it remains as aresidual film clinging tightly to the drum surface, and passes betweenthe. knife and the.

drum surface. As the drying proceeds, the continuous abrasion by thefilm of material dulls the knife very quickly and the residual filmwhich the knife does not remove inprea ses'rapidly in thickness, therebygreatly obstructing the flow of heat required for the drying,-this inturn reduced the capacity of the dryer and increases the moisturecontent of that part of the dry 'product which the knife is stillcapable of removingr On products such as sodium sulphate, etc, theconditions as outlined will continue to grow worse until shortly a pointis reached at which the dry ing has to be discontinued.

On some inorganic salts, the residual film builds excessive pressure ofthe knife and possibly increased expansion of the drum, caused bythethis high temperature, such film at intervals will lose its grip on thedrum surface and will be re moved entirely or irregularly in spots.Should the material be sensitiveto heat, the residual film periodicallyremoved would, of course, be overheated and inferior in quality.Moreover, regardless of whether the material is sensitive up to acertainthickness, then, on account of the to heat or not, the. textureof the dry product would not be uniform and would vary from a finepowder to large flakes or hard scales which would require a specialgrinding operation. The capacity and moisture content of the dry productwould vary as the residual film would be in process of building up or beremoved. The power requirements for the removal of the product from thedrums will be excessive and the results as a whole far fromsatisfactory. If a heavier or stiffer knife blade is selected or theknife placed at a steeper angle in relation to the drum surface in aneffort to cutintn the residual film, this may result in the scoring ofthe drum surface ordeep go ng of the metal which may be sufiiciently secue to render the drum unfit for further use.

In. the type of dryers known as doubledrumdryers, the drums are setclose board at each end of the drums forming a reservoir on top of, andbetween the drums, forholding' together, with an end andpre-concentrating the liquid to be dried. The drums revolve ingadownward direction I through this reservoir, the'clearance between thetwo drums determining. the thickness of the mm on the surface. On thistypeof dryer any residual film, whether covering. the 'drum surfaceentirely and uniformly or irregularly in spots, will seriously interferewith the application of a liquidfilm of uniform thickness on the drumsurface. The original clearance between the drums generally is between.030" and .040". The residual layer on the drums soon builds u apointwhere this clearance will be closed altogether as a result erted on theresidual films at a point where they meet between the drums.

This pressure becomes greater as the residual film increases in thick ofwhich pressure will be ex-' ness and tends to crush and crack the hardfilm I which makes it possible for the knife to remove all or part ofthis heavy coating, resulting in an increased clearance and leakagebetween the drums in, places where the coating has been removed. Theexcessive pressure between thedrums caused by the residual filmremaining .on

plated with chromium s. This drum is' suitably Joumaled in end standardsI and is provided with a steam inlet 8 and also with suitable manner(not shown) for removing the condensate. The

the drum surfaces, will in a very short time disiron or steel does notappear to depend entirely. upon the size of the grain but it appearsthat the tenacity with which the metal retains a film of inorganic saltor sludge is due to the chemical composition of the metal, its internalgrain structure and possibly the oxidation of the surface, which aloneor in combination form a perfect anchorage for inorganic salts.

I have found that if inorganic salts or sludges I are dried upon drumshaving a grain structure which provides a smooth metallic surface, thesedifliculties do not occur. The film of dry material no longertenaciously adheres to the drum surface and can be removed by thedoctor-blade without applying excessive pressure. Consequently, heattransfer is improved, the drum speed can "be increased, a high capacityis obtained, moisture content of the dried product beof the product, andno contamination 'of the product. Moreover, since the knifepressurecan-be elieved, the wear of the lmives is greatly reduced, powerconsumption is greatly decreased,

liquid or pasty inorganic'salt or sludge. to be dried can be applied tothe chromium plated surface 6 of the drum 5 in any one of a number ofdifferent ways but as shown the drum is arranged to dip into a pan 9which iskept filled with a bath [0 ofthe material to be dried. The drumis rotated in'the direction of the arrow and as it rotates the fllnr ofliquid picked up from the pan 9' dries and is scraped off by alongitudinal knife or doctor blade- II which is preferably provided withmeans (not shown) for adjusting its pressure against the chromium platedsurfaceof the drum 5. The dried material scraped from the drum by theknife ll falls into a trough i2 from which it is conveyed by means .of ascrew conveyor l3. When the inorganic salts are to be dried; the sameare fed to the heated drying drum in the form of a solution, slurry orsludge.

When the inorganic salts are to be cooled or' flaked the salts are-fusedand may or may not contain water and the drum is cooled. In the doubledrum dryer shown in Fig. 2 tw parallel drums I are provided and eachis-'composed of a welded seamless drawn or cast steel or cast iron'bodyhaving its outer cylindrical face plated with chromium 6. As in the formshown in Fig. 1 each drum is provided with a steam inlet 8 and suitablemeans for' removing the concomes uniform, there is no injury to thequality 'densate. The drums are supported for continuous rotation in thedirection of the arrows on a frame It which also supports an end plateIt at opposite ends of the drums 5. These end plates enclose the valleybetween the closely spaced drums, and the material to be dried is heldin a I valley ortrough formed between the drums and and the danger of.the knives damaging the drums is removed. I i

It is thereforethe object of this invention to provide a processofdrying inorganic salts and sludges which consists in drying thesematerials upon a smooth metallic surface from which they with chromium.

pheric single drum can be readily scraped, thereby'avoiding' thedifficulties encountered in drying these materials on steel or cast irondrums.

In the accompanying drawing:

Fig. 1 is a vertical transverse section through i a simple form ofsingle drum atmospheric dryer adapted for use in carrying out myimproved process, and showing, in a simplified manner the manner inwhich the material is applied to the drum and scraped therefrom andshowlna the drum made of steel or cast iron and plated Fig. 2 is asimilar section through a simple form of atmospheric double drum dryerand showing the two drums also made of cast iron and plated withchromium.

Fig. 3 is an enlarged fragmentary crosssection of a steel or cast irondrum plated withchromium. Fig. 4 is a similar frmmentary cross sectionof a steel or cast iron drum having a stainless steel sleeve appliedthereto. v

The apparatus shown forcarrylng out the present process is shown asembodied in a singleand a double drum atmospheric dryer, although itwill be understood that the invention is\also applicable to vacuum drumdryers. .Inethe atmosdryer illustrated in a simpliiied manner in Fig.ii, the drum l is made of welded, seamless drawnor cast steel or castiron in the usualmannsr, and hssits cylindricalface 'ever.'for generalpurposes, pla

between the end plates l6, .As the drums rotate they'pick up a film ofthis material and when thisfllm is dried it is scraped oiliby doctorblades or scrapers "which preferably also have means (not shown) for theadjustment of their pressure upon the surface of the drum. The driedmaterial falls into'troughs 18 from which it is conveyed away by meansof screw conveyors i9.

Instead of plating the drums I with chromium,

as shown in Figs. 1, 2 and 3 a sleeve of stainless steel 20 can beapplied to the body 5 of the drum, as shown in Fig. 4. Stainless steelhas an austenitic grain structure as compared with chromium which inplated construction is generally considered to be amorphous. Drums madecompletely of stainless steel have been found satisfactory but forreasons of economy it is preferable to make the drums of-steel or ironand place the stainless steel vsleeve around these drums. Howthe drums Iwith chromium has been found to be the most satisfactory as hereinaftermore fully pointed out.

Until recently only steel, cast iron and bronze were available forcommercial drums for atmospheric or vacuum dryers. Although bronzeoflers a considerably better surface than steel or iron it is subject tothe same difllculties to a lesser degree and has the additionaldisadvantase of comparative softness andof being easily, damaged by theknives. Nickel, Monel metal and aluminum are also comparatively .softand ofler diiilculties in making. sound m inss of the required size.Stainless steel and chromium plated. welded.

seamless drawmorcast steel or iron drums have proved to givesatisfactory Stainless steel 2,084,599 can be used in the form of ahomogeneous drum I used in connection with a stainless steel drumsurface to prevent scoring the wear of these knives again becomesexcessive. Cast iron and steel alloys of an austeniticgrain structuresuch as nickel-iron and. nickel-copper-iron alloy, known commercially asNi-resist, have been found to be an improvement but at present sufierfrom the same disability of comparative softness. Both of these alloysand stainless steel form a homogeneous solid solution in the solidstate. Nickel plating does not ofier much promise because of thesoftness of the nickel and the danger of breaking or peeling of thisplating. However, improved knives which require less tension and abetter electro-deposit of nickel may render a nickel plated drumentirely satisfactory.

The chromium plating of welded, seamless drawn or caststeel or irondrums hasproved entirely satisfactory as it forms a hard, smooth coatingwhich isharder than the steel or iron surface and is not subject toatmospheric corrosion. Special steel, iron, nickel or bronze alloys ofsuitable hardness, internal grain structure and surface condition alsoofier a possibility.

With chromium plated drums drying sodium sulfate it has been possible toincrease the speed from 2 R. P. M. with cast iron drums to 10 R. P. M.obtaining an increase in capacity of 60% over that obtained on cast irondrums. Moreover the moisturecontent of the dried product was reducedfrom 1.6-1.8% on the cast iron drums to from 02-03% on stainless steelor chromium plated drums.

Sodium sulfite sho vs an increase in capacity of in favor of thechromium plated drum, at approximately the same drum speed. However, itis possible with the chromium plated drum to double the speed and obtainan increase in capacity of 170%. d

Sodium phosphate shows the same increase in capacity and a reduction inpower consumption from 26 horse power for the cast iron drums to lessthan 15 horse power for the chromium plated drums. This result wasobtained in a commercial drying installation of much larger size thanthe experimental dryer on which the other tests were made.

Tests have indicated that it is apparently more satisfactory to chromiumplate steel drums than cast iron drums. Due to the lack of sufiicienthardness of steel which can be used in making drying drums, the use ofchromium plated steel drums appears to be particularly applicable tosingle drum atmospheric and vacuum dryers for drying inorganic salts andsludges.

The smooth surfaced drums forming the subject of this invention can alsobe advantageously used as flaking or cooling drums in flaking inorganicsalts and sludges which materials adhere very tenaciously to cast irondrums and can only be removed from the drum surface by using greatpressure on the knife. By providing a smooth metallic surface, as bychromium plating the flaking or cooling drums, the same advantageousresults are obtained in flaking these materials.

The present invention therefore provides a process for drying inorganicsalts or sludges on a smooth surface to which the material will not ad-'here tenaciously and can be readily scraped off by knives which areadjusted to bear with only relatively slight pressure against the dryingsurface. This avoids all of the difficulties referred to with referenceto iron or steel drums on which, unless excessive knife pressure isused, a film of the dried material will adhereito the drum surface,build up in the form of a residual film or layer of dried material,reduce the. efliciency of the dryer, slow up the drying process, requiregreater heat and result in a product which is liable toube contaminated.

I claim as my invention:

The method of drying a solution of an inoranic salt selected from agroup comprising sodium sulfate, sodium sulfite, ammonium phosphate,sodium phosphate, sodium carbonate, sodium chloride and potassium salts,which consists in applying a coating of a solution of the nmx J. VANMARLE.

